I'm not sure specifically as I haven't read it. But generally, a graphic novel combines visual art with storytelling. 'Sonne und Beton' might be about various themes like urban life (since 'beton' means concrete in German, could imply an urban setting) and perhaps something related to the sun which could symbolize hope, warmth or new beginnings in the context of the story.
Since I'm not familiar with this graphic novel, it's difficult to determine the main characters. However, given the title, there might be characters who are either trying to escape from a cold, concrete - like existence (represented by 'beton') and are drawn to something symbolized by the sun, like hope or a better life. Or perhaps there are characters who are part of a world where the sun and the concrete play important roles in their daily lives and personalities.